Federal Reserve Chairman Ben S. Bernanke and then-New York Fed President Timothy Geithner told senators on April 3, 2008, that the tens of billions of dollars in “assets” the government agreed to purchase in the rescue of Bear Stearns Cos. were “investment-grade.” They didn’t share everything the Fed knew about the money.

The so-called assets included collateralized debt obligations and mortgage-backed bonds with names like HG-Coll Ltd. 2007-1A that were so distressed, more than $40 million already had been reduced to less than investment-grade by the time the central bankers testified. The government also became the owner of $16 billion of credit-default swaps, and taxpayers wound up guaranteeing high-yield, high-risk junk bonds.

By using its balance sheet to protect an investment bank against failure, the Fed took on the most credit risk in its 96- year history and increased the chance that Americans would be on the hook for billions of dollars as the central bank began insuring Wall Street firms against collapse. The Fed’s secrecy spurred legislation that will require government audits of the Fed bailouts and force the central bank to reveal recipients of emergency credit.